Question: Multiply the following complex numbers, marked as blue dots on the graph: $(3 e^{11\pi i / 6}) \cdot (2 e^{2\pi i / 3})$ (Your current answer will be plotted in orange.)
Solution: Multiplying complex numbers in polar forms can be done by multiplying the lengths and adding the angles. The first number ( $3 e^{11\pi i / 6}$ ) has angle $\frac{11}{6}\pi$ and radius $3$ The second number ( $2 e^{2\pi i / 3}$ ) has angle $\frac{2}{3}\pi$ and radius $2$ The radius of the result will be $3 \cdot 2$ , which is $6$ The sum of the angles is $\frac{11}{6}\pi + \frac{2}{3}\pi = \frac{5}{2}\pi$ The angle $\frac{5}{2}\pi$ is more than $2 \pi$ . A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $\frac{5}{2}\pi - 2 \pi = \frac{1}{2}\pi$ The radius of the result is $6$ and the angle of the result is $\frac{1}{2}\pi$.